It will need to be a 2-step process setup with the IdentitySync Dataflows where the first-step imports the subscription using either the accounts.importFullAccount REST or accounts.importLiteAccount REST API methods, and the second step calls the accounts.setAccountInfo REST API method to actually update the subscription and send the email. You can discuss possible approaches further with SAP's expert/consulting services team who would be the best point-of-contact to help you review your exact requirements and tweak the dataflow to suit your business needs. The gist of it is that you can use the "record.evaluate" dataflow component to setup a custom script that will enable you to import all subscriptions on the import step with:
, and then on the setAccountInfo step change the subscriptions that the user has subscribed to back to:
As a side note, it's also important to be considerate of the general do's and don'ts for sending email blasts (see here). We would strongly recommend setting-up your own SMTP relay to send the emails instead of through Customer Data Cloud's default relay which will is meant for testing purposes and will not pass DMARC validation, and discussing further with your mail provider if they have any concerns with the volume of email you are looking to send, and see if they can provide any recommendations on the strategy and best-practices.